atomthreads/ports/stm8/README-IAR

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Library:      Atomthreads
Author:       Kelvin Lawson <kelvinl@users.sf.net>
Website:      http://atomthreads.com
License:      BSD Revised

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STM8 PORT - IAR COMPILER

This folder contains a port of the Atomthreads real time kernel for the
STM8 processor architecture. These instructions cover usage of Atomthreads
with the IAR Embedded Workbench compiler (EWSTM8).

All of the cross-platform kernel code is contained in the top-level 
'kernel' folder, while ports to specific CPU architectures are contained in
the 'ports' folder tree. A port to a CPU architecture can comprise just one
or two modules which provide the architecture-specific functionality, such
as the context-switch routine which saves and restores processor registers
on a thread switch. In this case, the kernel port is split into two files:

 * atomport.c: Those functions which can be written in C
 * atomport-asm-iar.s: The main register save/restore assembler routines

Each Atomthreads port requires also a header file which describes various
architecture-specific details such as appropriate types for 8-bit, 16-bit
etc variables, the port's system tick frequency, and macros for performing
interrupt lockouts / critical sections:

 * atomuser.h: Port-specific header required by the kernel for each port

A few additional source files are also included here:

 * tests-main.c: Main application file (used for launching automated tests)
 * uart.c: UART wrapper to allow use of stdio/printf()
 * stm8s-periphs/*.*: Peripheral drivers as delivered by ST (no changes
     to distributed code).

Atomthreads includes a suite of automated tests which prove the key OS
functionality, and can be used with any architecture ports. This port
provides an easy mechanism for building, downloading and running the test
suite to prove the OS on your target.

The port was carried out and tested on an STM8S105C6 running within an
STM8S-Discovery board, and supports both the Cosmic and IAR compiler tools.
It is possible to use it with other processors in the STM8 range, as well
as other hardware platforms and compilers, with minimal changes. Platform
and  compiler specific code has been kept to an absolute minimum. This
README covers usage of Atomthreads with the IAR compiler. Instructions for
users of the Cosmic compiler are available in README-COSMIC.


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PREREQUISITES

The port works out-of-the-box with the IAR compiler tools for building.
Applications are generated in ELF format and can be programmed and debugged
using the IAR Embedded Workbench GUI or the free STVP (visual programmer
tool). At this time there does not appear to be a command-line programmer
application suitable for use with STM8.

IAR Embedded Workbench for STM8 is a Windows-only application. For
users of other operating systems the IAR tools may work in environments
like Wine, but the USB programming tools are less likely to be supported.
Embedded Workbench for STM8 can, however, be run successfully within a VM
such as VirtualBox, including USB download and debug.

The core software prerequisites are therefore:
 * IAR Embedded Workbench STM8

Use with alternative compiler tools will require some modification, but you
can easily replace the EWSTM8 IDE by your own favourite programmer if
required (e.g. STVP).


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BUILDING THE SOURCE

You may build Atomthreads using whichever build environment you desire. For
your convenience we provide both a ready-rolled Makefile-based build system
and an Embedded Workbench (EWSTM8) project. The EWSTM8 project permits easy
building, programming and debugging, but does not easily support building
a wide range of application builds within the same project, which is
useful for building the numerous automated tests. For the automated tests
you may find it easier to use the Makefile which automatically builds all
automated tests.


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BUILD VIA EWSTM8 PROJECT

For building applications using the EWSTM8 IDE you can use the sample 
project file atomthreads-sample-iar.ewp. This builds a sample full
application which runs the "sem1" automated test. Applications can be
downloaded directly to the target hardware (e.g. STM8S-Discovery) and run
via the integrated debugger. You can start the application running, and
confirm that the LED flashes once per second (if running on an
STM8S-Discovery) to ensure that the test has passed. 

This is also a good starting point for building your own applications:
simply modify the file tests-main.c which starts the test application.
You can run any of the other automated tests by replacing the file sem1.c
within the project by another of the tests within the atomthreads tests
folder. This is rather painful using a GUI interface due to the large
number of test files, and you may prefer to use the Makefile-based system
instead which builds all automated tests in one command.


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BUILD VIA MAKEFILE

A Makefile is also provided for building the kernel, port and automated
tests. This is particularly useful for building the automated tests
because many different independent applications need to be built which is
not easily achieved within the EWSTM8 environment.

For a Windows system you can obtain a Make application suitable for use
with the IAR compiler from:

 * http://www.cosmic-software.com/comp_utils/GNU_Make.zip

Assuming you install the above into C:\Program Files\GNU_MAKE, you
should set up your environment variables as follows:

 * set PATH=%PATH%;C:\Program Files\GNU_MAKE;C:\Program Files\IAR Systems\Embedded Workbench 6.0\stm8\bin
 * set MAKE_MODE=DOS


The full build is carried out using simply:

 * make -f iar.mak

All objects are built into the 'build-iar' folder under ports/stm8. The
build process builds separate target applications for each automated test,
and appropriate .elf or .s19 files can be found in the build folder ready
for downloading to and running on the target. Because of the limited
resources on the STM8, and the large amount of automated tests, each test
is built and run as a separate application.


All built objects etc can be cleaned using:

 * make -f iar.mak clean


The Atomthreads sources are documented using Doxygen markup. You can build
both the kernel and STM8 port documentation from this folder using:

 * make -f iar.mak doxygen


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PROGRAMMING MAKEFILE-BUILT APPLICATIONS TO THE TARGET DEVICE

When developing within EWSTM8, programs can be downloaded directly to the
target. If, however, you are building applications separately using a
Makefile or similar, then you are not able to program the application
using EWSTM8. None of the tools delivered by ST appear to be designed to
cater for those who build applications externally, but it is possible using
STVP.

The following development workflow can be used (note that these settings
apply to the STM8S-Discovery):

 * Build app using Makefile.
 * Open STVP and configure to use Swim ST-Link for CPU STM8105C6.
 * Open application .s19 file and program using "Program All Tabs".

Unfortunately STVP does not have a command to reset and start the CPU
running, but it can be forced into doing so by reconfiguring the
programmer:

 * Select "Configure ST Visual Programmer" from the Configure menu.

Your application should now be programmed and running.

If you wish to program and run another application then you can open and
program it in STVP, then use the Configure menu again to reset the
device and start it running.

Other programming tools may exist but are not apparent in the toolset
delivered for use the STM8S Discovery platform.


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STM8S-DISCOVERY SPECIFICS

There are very minimal board-specific aspects to the STM8 port so it is
trivial to run Atomthreads on other STM8 platforms.

The test applications make use of a LED to indicate test pass/fail status.
This is currently configured to use a bit in GPIOD, which on the Discovery
board maps to the board's only LED. You may change the port and register
bit in tests-main.c to utilise a different pin on other hardware platforms.
You may also completely omit the LED flashing in the test application if
you prefer to use the UART for monitoring test status.

The test applications also make use of the UART to print out pass/fail
indications and other information. For this you should connect a serial
cable to the Discovery board via the external pin connectors. Use of
a UART is not required if you prefer to use the LED or some other method
of notifying test pass/fail status.

To connect a serial cable to the Discovery you will need to connect to
the following pins on the external connectors:
 Vcc: CN2 pin 8
 GND: CN2 pin 7
 UART TX: CN4 pin 10 (connect to RX at the PC end)
 UART RX: CN4 pin 9 (connect to TX at the PC end)
Note that the board uses TTL levels so you may need to use a level
converter. External level converters may need to be powered using
a Vdd of 5v, which can be achieved by positioning JP1 on the Discovery.

The STM8 device on the Discovery only offers UART2. If you are using a
different device or wish to use an alternative UART then you must change
the stm8s_conf.h file.

If you are using a CPU other than the STM8S105C6 you should change the
PART macro from "STM8S105" to your target CPU. This can be changed in the
iar.mak Makefile. If you are using the EWSTM8 project it should be
changed in the project C/C++ Compiler Preprocessor settings for both Debug
and Release builds, and you must also change the target device in the
project's "General Options". You may also wish to enable any CPU
peripherals which you wish to use in the stm8s_conf.h file.


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RUNNING THE AUTOMATED TESTS

Atomthreads contains a set of generic kernel tests which can be run on any
port to prove that all core functionality is working on your target.

The full set of tests can be found in the top-level 'tests' folder. The
Makefile builds each of these tests as independent applications in the
'build' folder. Run them individually using the STVP process described
above. For example to run the 'kern1.c' test use STVP to program and run
it.

You may also build the tests using the EWSTM8 project, but to run each
different test you must manually remove the previous test module (e.g.
kern1.c) and replace it with one of other tests, which can be quite time
consuming compared to building all tests in one command via the Makefile.

To view the test results, watch the LED on the STM8S-Discovery. This will
flash once per second if the test passed, and once every 1/8 second if the
test failed.

If you wish to use the UART, connect a serial debug cable to your target
platform (defaults to 9600bps 8N1). On starting, the test applications
print out "Go" on the UART. Once the test is complete they will print
out "Pass" or "Fail", along with other information if the test failed.

Most of the tests complete within a few seconds, but some (particularly
the stress tests) can take several seconds, so be patient. 

The full suite of tests endeavours to exercise as much of the kernel code
as possible, and can be used for quick confirmation of core OS
functionality if you ever need to make a change to the kernel or port.

The test application main() is contained in tests-main.c. This initialises
the OS, creates a main thread, and calls out to the test modules. It also
initialises the UART driver for use by stdout.


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WRITING APPLICATIONS

The easiest way to start a new application which utilises the Atomthreads
scheduler is to base your main application startup on tests-main.c. This
initialises the OS, sets up a UART and calls out to the test module entry
functions. You can generally simply replace the call to the test modules by
a call to your own application startup code.

Projects developed within EWSTM8 can be started using the sample project
atomthreads-sample-iar.ewp. If you wish to create your own EWSTM8 project
from scratch, then you should ensure you change the project settings for
both Debug and Release builds as follows:

* General Options -> Target -> Device: CPU part (e.g. "STM8S105C6")
* C/C++ Compiler -> Diagnostics: Suppress "Pa050"
* C/C++ Compiler -> Preprocessor -> Defined Symbols: 
            CPU part (e.g. "STM8S105")
            Thread stack-checking if required ("ATOM_STACK_CHECKING")
            For example "STM8S105, ATOM_STACK_CHECKING"
* Assembler -> Diagnostics: Suppress "Pa050"
* Repeat above for Debug and Release projects (you may want to 
  disable ATOM_STACK_CHECKING for Release builds).

Other options you may wish to change:

* Tools -> Options -> Editor -> EOL Characters: "Preserve". This preserves
  the line endings, bearing in mind that the Atomthreads kernels works
  with many host operating system toolchains.
* Options -> Debugger -> "ST Link" (e.g. for STM8S Discovery)

Add the .C and .S modules from the following folders:
* kernel
* ports/stm8
* ports/stm8s-periphs

Set include paths as appropriate.


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RAM FOOTPRINT & STACK USAGE

The Atomthreads kernel is written in well-structured pure C which is highly
portable and not targeted at any particular compiler or CPU architecture.
For this reason it is not highly optimised for the STM8 architecture, and
by its nature will likely have a higher text and data footprint than an
RTOS targeted at the STM8 architecture only. The emphasis here is on
C-based portable, readable and maintainable code which can run on any CPU
architecture, from the 8-bitters up.

A good rule of thumb when using Atomthreads on the STM8 architecture is
that a minimum of 1KB RAM is required in order to support an application
with 4 or 5 threads and the idle thread. If a minimum of approximately
128 bytes per thread stack is acceptable then you will benefit from the
easy-to-read, portable implementation of an RTOS herein.

The major consumer of RAM when using Atomthreads is your thread stacks.
Functionality that is shared between several kernel modules is farmed out
to separate functions, resulting in readable and maintainable code but
with some associated stack cost of calling out to subroutines. Further,
each thread stack is used for saving its own registers on a context
switch, and there is no separate interrupt stack which means that each
thread stack has to be able to cope with the maximum stack usage of the
kernel (and application) interrupt handlers.

Clearly the stack requirement for each thread depends on what your
application code does, and what memory model is used etc, but generally
you should find that 128 bytes is enough to allow for the thread to be
switched out (and thus save its registers) while deep within a kernel
or application call stack, and similarly enough to provide stack for
interrupt handlers interrupting while the thread is deep within a kernel
or application call stack. You will need to increase this depending on
what level of stack the application code in question requires.

At this time the maximum stack consumed by the test threads within the
automated test modules is 85 bytes of stack, and the main test thread has
been seen to consume 193 bytes of stack. At this time the queue9 test is
the largest consumer of test thread stack (85 bytes) and the sem8 test
consumes the largest main thread stack (193 bytes). If your applications
have large amounts of local data or call several subroutines then you may
find that you need larger than 128 bytes.

You may monitor the stack usage of your application threads during runtime
by defining the macro ATOM_STACK_CHECKING and calling
atomThreadStackCheck(). This macro is defined by default in the EWSTM8
Debug project so that the automated test modules can check for stack
overflows, but you may wish to undefine this in your application
when you are happy that the stack usage is acceptable. Enabling 
ATOM_STACK_CHECKING will increase the size of your threads' TCBs
slightly, and will incur a minor CPU cycles overhead whenever threads are
created due to prefilling the thread stack with a known value.

With careful consideration and few threads it would be possible to use
a platform with 512 bytes RAM, but not all of the automated test suite
would run on such a platform (some of the test modules use 6 threads: a
main thread together with 4 test threads and the idle thread).


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INTERRUPT HANDLING

Interrupt handlers use the stack of the thread which was running when the
interrupt occurred. If no thread rescheduling occurs during the ISR then
on exit from the ISR any data stacked by the ISR on the thread's stack is
popped off the stack and execution of the thread resumes. If a reschedule
during the ISR causes a context switch to a new thread, then the ISR's
data will remain on the thread's stack until the thread is scheduled back
in.

Interrupt priorities (via the ITC_SPRx registers) are left in their
default power-on state, which disables interrupt nesting. Kernel changes
may be required to support interrupt nesting.

Note that the STM8 programming manual currently describes the following
feature:

 "Fast interrupt handling through alternate register files (up to 4
  contexts) with standard stack compatible mode (for real time OS
  kernels)"

This feature was implemented by ST in the core but has to date never been
included in any STM8 products. If it is included in future products then
you will need to put the device in the stack compatible mode described.


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WRITING NEW INTERRUPT HANDLERS

All interrupt handlers which will call out to the OS kernel and potentially
cause a thread switch must call atomIntEnter() and atomIntExit(). An
example of this can be seen in the timer tick ISR in atomport.c.

You may also implement fast interrupt handlers in the system which do not
call atomIntEnter()/atomIntExit(), however these ISRs cannot perform OS
functions such as posting semaphores or effecting a thread switch.


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IAR COMPILER VIRTUAL REGISTERS

The STM8 has only very few CPU registers, so the IAR compiler augments
them with sixteen "virtual" registers, which are simply locations in fast
memory. These registers are called ?b0 to ?b15.

The Atomthreads context switch for IAR/STM8 takes advantage of the fact
that all CPU and most virtual registers are automatically saved on the
stack by the compiler when calling out to C functions (and even then only
if necessary). Only the virtual registers ?b8 to ?b15 are expected to be
preserved by called functions, so these are the only registers that
callers to the context switch routine will not automatically save if
necessary.

For cooperative context switches (where a thread calls an OS kernel
function to schedule itself out), most registers will therefore already
be saved on a thread's stack if necessary. Only ?b8 to ?b15 actually have
to be saved in the context switch routine, making cooperative switches
potentially very cheap if few registers must be preserved.

For preemptive switches (where an ISR has interrupted a thread and wishes
to switch to a new thread), the interrupt handler prologue automatically
saves all CPU registers (actually done automatically by the CPU) and the
virtual registers ?b0 to ?b7. Still only the registers ?b8 to ?b15 have to
be saved by the context-switch routine, but in this case ?b0 to ?b7 and the
CPU registers are always saved on the thread's stack by the ISR prologue.
This is because the ISR has no knowledge of what registers the interrupted
thread was using, so we cannot take advantage of the potential for saving
fewer than the full set of registers that we achieve with cooperative
switches.


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